28 



The portions a^h^c^ of the short leg are filled up to a point at about A 

 with colored water, and then above this with kerosene or some other 

 liquid of nearlj^ the same density as water and with which it does not 

 mix. It is plain that when the level of the mercury in the short leg 

 changes, the meniscus separating the oil from the water in the small- 

 bore tube will be seen to change its position by an amount enlarged 

 in proj)ortion to the relation between the areas of the cistern and the 

 small tube. 



67. General comments on Trmgnifi/inff harometers. — The special ba- 

 rometers described above are interesting and in many respects curious, 

 but they can not be regarded as anything more than philosophical 

 toys and curiosities. It is impracticable in any of the magnifying 

 instruments to determine the absolute pressure with great precision, 

 for the expedients of magnification introduce sources of both constant 

 and accidental errors that affect the results to a magnified extent, so 

 that even less precision generally results than is attained with well- 

 made mercurial barometers of the simple pattern. 



IV. Barographs, or Continuotjsly Recording Barometers 



68. The barograph is a form of barometer with the addition of 

 parts by which a continuous record of the barometric oscillations is 

 traced upon a sheet of moving paper or obtained by photographic 

 processes upon sensitized plates. Many different forms of apparatus 

 have been devised, nearly all of which are more or less elaborate, in 

 general, but not correspondingly accurate. In most cases the chang- 

 ing temperature to which the instruments are subjected introduces 

 small errors, and the mechanisms effecting continuous registration 

 either obstruct the free action of the barometer proper or do not 

 transmit to the record sheet a trace representing exactly the original 

 fluctuations of pressure. The absolute value of the pressure indi- 

 cated by the position of the tracing point is also subject to uncertain- 

 ties, and while changes of pressure are shown with greater or less 

 accuracy, it is impossible, even with the best instruments, to record 

 the absolute pressure with a precision equal to that of an eye reading 

 of a standard barometer. In general, therefore, the indications of 

 automatic instruments are checked and corrected by reference to occa- 

 sional eye readings of a standard barometer. 



69. It hardly requires to be said that the mercurial baragraph is 

 more reliable and gives more accurate results than those of the 

 aneroid type. Many ingenious mechanisms are employed to effect 

 the continuous registration, thus: 



(1) In some cases this is accomplished by directly photographing 

 on a moving sheet of sensitized paper or a moving plate the changing 

 positions of the summit of a barometric column, notably the case at 

 the Kew Observatory, England, 



(2) In other forms, a float resting upon the mercury in the open leg 

 of a siphon barometer communicates the motion directly to a lever 

 which carries the recording pen at its end and produces the record on 

 a magnified scale. It is difficult to overcome the friction of the pen 

 and magnifying levers sufficiently to obtain correct records ; the con- 

 struction in some forms is therefore modified and clock movements or 

 electromagnets are caused to perform the real labor of producing the 

 record. Under this category we have such cases as follow : 



